1. Field of the Invention
The present invention relates to the field of document cameras, and in particular to indicating an effective field of view for a document camera. The invention is especially suitable for portable (e.g. handheld) document cameras.
2. Description of Related Art
A document camera is defined herein as a digital camera having a digital resolution sufficient to enable processing of a captured document image by an optical character recognition (OCR) algorithm. The resolution is therefore equivalent to at least 100 dpi (dots per inch) of the original document.
For capturing an image of a page, for example, lying flat on a table, most digital cameras are designed for capture while a user is standing, and require the user to look through a viewfinder to select the field of view. It is therefore awkward for a seated user to look through a viewfinder to capture a page, as this requires an unnatural leaning posture to look down on to the page. It can also be awkward for people who wear glasses.
Although some digital cameras provide a display for visual feedback, this increases the cost of the camera, and is usually fixed on the back of the camera, which again makes it hard to use for a seated user. Some cameras have a foldout video display. However, these can make hand-eye co-ordination awkward, as the view co-ordinate system is not the same as the user""s view co-ordinate system. Furthermore, all electronic displays require that the camera be kept in focus while the user moves it, in order to provide satisfactory visual feedback.
A further problem with electronic displays is that, if a user wishes to capture a small section of a document in a zoomed mode, he must work out exactly where his current view of the document is in relation to the area he wishes to acquire.
It has been suggested in the field of general cameras, to provide a camera with a projector for projecting a light source on to the scene to be captured, the light source providing boundary markers indicative of the field of view. This can remove the need for a user to look through a viewfinder, as the field of view is indicated in the object scene. The light source may, for example, be a semiconductor laser. Reference is made to U.S. Pat. Nos. 5,589,905; 5,500,702 and 5,485,235, the contents of which are incorporated herein by reference. However, these arrangements are not directed at document cameras and do not address any of the problems specific to this field of image capture.
Further reference may be made to U.S. Pat. Nos. 5,189,463; 5,628,034; 5,546,156; 5,543,889; 5,666,577 and 5,835,241, and to UK Patent Application GB-A-2252836. The contents of these documents are also incorporated herein by reference.
It would be desirable to provide a document camera which can be used more conveniently by a seated user, without the need for a viewfinder.
It would also be desirable to indicate variable fields of view for a document-imaging camera.
Broadly speaking, a first aspect of the present invention is to provide a document camera with a projector for projecting an indicator onto an object scene to be captured, the indicator indicating the field of view of the camera.
This can enable the operator to view the field of view on the document itself, without having to look through a camera viewfinder, which might be inconvenient.
In a preferred form, the projector is operable to project the field of view indicator to have a variable size and/or position. The camera includes a controller for controlling the size and/or position of the projected field of view indicator, to vary the field of view of the camera.
In a particularly preferred form, the camera comprises an image processor to process an at least partly captured image to detect an optimum capture zone for the document. The capture zone corresponds to whole sections or regions of the document which fall entirely within the available field of view of the camera. Any sections which are not entirely within the available field of view are deemed not to be included in the captured image, and the projected field of view indicator is controlled to indicate only the capture zone. When the image is to be captured, the camera zooms in to capture the image region corresponding to the capture zone at maximum (or at least an increased) resolution.